This invention relates to steam turbine buckets generally and to the incorporation of a tip leakage loss reduction feature in the thermal barrier coating applied to the bucket tip.
The radially outer tips of gas turbine buckets serve in a hostile environment of both high temperature and high rotationally-induced stress. The life of parts subjected to these conditions is typically limited by low-cycle fatigue (LCF) and creep considerations. In accordance with conventional practice, a tip cap is welded to the bucket as part of a current manufacturing process for hot gas path sealing purposes. The addition of a conventional metal seal to the existing tip cap increases the thermal gradient at the tip, however, and therefore degrades the LCF and creep life. In prior art buckets, this is overcome by employing film cooling in the bucket tip region. In closed-loop steam-cooled turbine bucket applications, however, airfoil film cooling cannot be practically applied, as there is only a single closed cooling circuit. A shroud covering the tip gap and cantilevered across the blade-to-blade gap, as typically applied on stage 2 and stage 3 buckets, is likewise not practical in the first stage due to LCF and creep considerations.
Air-cooled buckets typically have a metallic “squealer tip” feature; however, this approach is cast into the bucket which is not feasible for steam-cooled buckets. Thus, current closed-loop steam-cooled stage 1 buckets have no feature to impede fluid flow into the tip gap. As a result, leakage flow rolls into a vortex, causing a reduction in turbine efficiency by two means. First, the tip flow generates no lift, and contributes no power-producing torque on the turbine rotor. Second, the tip vortex mixes out with the surrounding flow downstream of the bucket, generating mixing loss.